(1) Field of the Invention
This invention relates to a process for producing an aliphatic isocyanate. More particularly, it pertains to a process for producing an aliphatic isocyanate from an aliphatic primary amine, carbon monoxide and molecular oxygen in a high yield.
(2) Description of the Prior Art
Heretofore, almost all aliphatic isocyanates have been produced by a reaction between aliphatic primary amines and phosgene. However, it is preferred to produce aliphatic isocyanates more simply without the use of phosgene because of several reasons, such as the high toxicity of phosgene, by-production of large quantities of corrosive hydrogen chloride, and, furthermore, the presence of difficult-to-remove hydrolyzable chlorine compounds.
On the other hand, it has been known that an aliphatic isocyanate is capable of being prepared at a yield of about 50% by reacting an aliphatic primary amine with carbon monoxide and a stoichiometric amount of palladium chloride (E. W. Stern et al, J. Org. Chem. Vol. 31, p. 596, 1966).
However, this method has been proven to be far from practical in industrial application, because an equivalent amount or more of sodium hydrogen phosphate is required as the dehydrochlorinating agent, and the reaction is not catalytic and needs a very long reaction time of from 48 to 60 hours during which time the palladium chloride is reduced to metallic palladium to stop the reaction. As a result the yield is low.
A similar reaction has also been reported in Japanese Patent Publication examined 650/1970 (and J. Tsuji et al, Chem. Commun., p. 828, 1966), but this method is also industrially impractical, for such reasons that stoichiometric amount of palladium chloride and allyl halide are employed, the reaction is not catalytic and the palladium chloride is converted to a .pi.-allyl complex to stop the reaction. In addition the yield is very low.
Thus, in order to avoid the problems mentioned above, the present invention provides a process for producing an aliphatic isocyanate from an aliphatic primary amine without the use of phosgene.